The long-term goals of this research are to determine the mechanisms in the brain with which disruptions of indispensable amino acid (IAA) homeostasis are recognized by omnivores, including humans. The specific aims are 1) to determine if IAA transport can activate neurons in vitro, and 2) to characterize these IAA transporters at the molecular level in vitro, and at the functional level, using appropriate transport inhibitors, via in vivo feeding studies in the rat. Disorders of IAA nutrition and metabolism are serious human health concerns, and often cause debilitating anorexia. This work is important for human health in that determination of the cellular responses to disruptions of IAA nutrition can lead to suggestions for medical intervention. The feeding (anorectic) responses to dietary IAA disproportion have been well studied, but it is not known how the brain acts in this recognition. We hypothesize that exchange of ions for IAA at the neuronal cell membrane alters the membrane potential and activates neurons, to provide a rapid transduction system mediating the detection of IAA status. First, deficient or replete cells will be exposed in vitro to media having deficient, imbalanced or excess IAA concentrations, and then subjected to transport studies, using established protocols. In vivo, rats will be given inhibitors of the transport systems determined in the in vitro studies. Reversal of IAA-induced anorexia in the rats will validate that system's role in the mechanism that provides recognition of IAA status.